Add replication slot propagation

[marcocitus]

When using logical decoding and adding a new node, you currently have to create a slot on the new node before rebalancing to make sure you do not miss any changes. This will prevent scenarios like automatic rebalancing.

It would be useful if replication slots can be propagated to new and existing nodes.

In this case, we cannot use pg_dist_object for bookkeeping, since replication slots do not have an OID. We might need a new metadata table that is synchronized across nodes.

Alternatively, we could automatically propagate all logical slots from the coordinator that do not belong to Citus when adding a node.

[onderkalaci]

In this case, we cannot use pg_dist_object for bookkeeping, since replication slots do not have an OID. We might need a new metadata table that is synchronized across nodes.

Maybe we keep an internal reference table if there is no OID involved?

(I have not thought enough about the implications, but throwing the idea here not to forget about it)

[marcocitus]

Then we'll need to figure out how to do schema changes on extension-owned Citus table during upgrades, which would be a good thing to figure out.